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Because of a fair number of reasons.
Northern Ireland:
She presided over a very dirty period of British history, in which we basically held political prisoners, tortured people, operated snatch squads etc. {To be fair, this wasn't a great departure from the previous government's policy, but Thatcher was more extreme.}
Economic policy:
On this point if you're a neo-liberal you'll probably disagree, but I think there's a good deal of evidence that financial service-based economies are inherently unstable, provide few jobs, and have a tendency towards concentrating wealth to the point of dysfunction. It's certainly the case that the heavy dependence of the UK economy on financial services has resulted in the UK remaining in reccession for far longer than almost anybody else. It's also the case that the industry-based economies have done far better.
Further, Thatcher basically destroyed a manufacturing economy which was, at the time, one of the most advanced in the world - in terms of both skills and institutions.
Socially:
She fractured the country, by shifting the economy towards financial services, the manufacturing-based towns {basically anywhere but London} became a gigantic rust-belt. And, since London was the only place that did financial services, it became incredibly rich. Some cities, like Coventry, have never recovered - turning from high-tech manufacturing bases to ghost towns overnight.
She also altered legislation so as to encourage people to get mortgages. This might sound like a good thing, until you consider what role it has to play today. By incentivizing house ownership, she essentially forced everybody to play as speculators in the property market. She 'tied the nation to bricks and mortar', making everybody more likely to make decisions based on improving their asset value, however NIMBYish and selfish these decisions might be. Further, speculation has a tendency of driving up prices, which has led to a situation where both rent and mortgages are incredibly expensive.

Essentially, she made a few people rich at the expense of the many, and backed it up with violence and appeals to our worst tendencies.

I don't see why people are happy that she's dead. I mean, it's not like she was damaging the country while she was a pensioner and I know people think she was a shit prime minister, but that doesn't mean she deserves to die.